1. Field of the Invention
The present invention relates to a reverse bias current supply system for a switching device for supplying a base reverse bias current to a switching semiconductor device such as a switching power transistor that is rendered conductive so that the switching device is interrupted.
2. Description of the Prior Art
When a power transistor which is conductive is interrupted or turned off, it is usual that a base reverse bias current (hereinafter referred to as I.sub.B2) is supplied to the transistor in order to obtain good switching characteristics. It is known that when the value of the base reverse bias current I.sub.B2 is increased, the safe operating region in which the base reverse bias is supplied to the power transistor is narrowed.
FIG. 1 is a graph illustrating an example of such a safe operating range. The vertical axis in FIG. 1 represents the collector current I.sub.C of a power transistor, and the horizontal axis represents the collector voltage V.sub.CE.
As can be understood from FIG. 1, when a collector current I.sub.C of 30 A is controlled in a case in which the base reverse bias current I.sub.B2 is 3 A, there is the possibility that the power transistor may be damaged if the surge collector voltage V.sub.CE is 300 V or less. Furthermore, if the base reverse bias current I.sub.B2 is increased to 6 A, there is the possibility that the power transistor may be damaged if the surge collector voltage V.sub.CE is 260 V or less to control the same collector current I.sub.C of 30 A. This means that the safe operating range is reduced from the solid line to the dash and dotted line.
As can also be understood from the above example, when the base reverse bias current I.sub.B2 varies in this manner, the safe operating range for the power transistor also varies. Accordingly, it is desirable to control the value of the base reverse bias current I.sub.B2 in such a way that good switching characteristics are obtained while the safe operating range is not narrowed.
Hitherto, however, limitations principally of cost and dimensions have generally prevented particular control of the base reverse bias current. For this reason, in the methods adopted to deal with variations in the base reverse bias current I.sub.B2 caused by such factors as variations in the power supply to the base reverse bias circuit and variations in component characteristics, the switching characteristics are checked at the minimum value of imaginary variations in the base reverse bias current I.sub.B2 under given working conditions, and also the safe operating range is checked at the maximum value of the imaginary variations in the base reverse bias current I.sub.B2. If both the checkings are cleared, the base reverse bias current is accepted. Consequently, because of the need to make a trade off between the switching characteristics of the power transistor and the safe operating range which depend on this base reverse bias current I.sub.B2, a switching power transistor has not necessarily been used under a condition of its maximum performance.